E. Hamada

Radiation effect on positronium formation in low-temperature polyethylene

Abstract The irradiation effect of γ-rays on polyethylene (PE) has been studied by positron annihilation lifetime spectroscopy (PALS). In the case of non-irradiated PE samples, at a low temperature below the glass-transition temperature, the intensity of the long-lived component of positronium, I3, increases due to an increase in the concentration of trapped electrons. However, the increase in I3 obtained in a few MGy γ-irradiated samples became very small due to the effect of induced radicals. It has been observed that the trapped electrons were affected by the intensity of the positron sources used for a PALS experiment.

Temperature and radiation effects on positronium formation

Abstract During positron annihilation lifetime spectroscopy (PALS) experiments of polymers, positrons emitted from the positron source induce radiation effects in the sample. Since these radiation effects can affect positronium (Ps) formation probability, PALS has some potential to be used as a new tool to study radiation effects. As a function of the contact time of the sample with the positron source, the Ps formation probability increases or decreases at low temperature or at room temperature, respectively. The decrease of the Ps formation probability at room temperature may be attributed to the accumulation of free radicals that can scavenge the electrons which could otherwise have combined with positron to form Ps. The increase at low temperatures may be attributed to the accumulation of trapped electrons that can be picked up by positrons to form Ps.

Pressure quenching of positronium in solid biphenyl

Abstract In solid crystalline biphenyl, ortho -positronium ( o -Ps), abundant at normal pressure, can be eliminated by application of a pressure of about 100 MPa. The lifetime of o -Ps shortens with the rise of pressure, as a result of diminishing of the free spaces in which it is trapped. However, the lifetime of para -Ps seems to increase, and at 80 MPa, it cannot be distinguished from that of free positrons. Another new effect is the reappearance of the long-lived component in the positron lifetime spectrum at high pressures. The same effect of positronium reappearance, although not so well pronounced, was found in biphenyl at low temperatures.

New system for a pulsed slow-positron beam using a radioisotope

To analyze the surface of polymers using positron-annihilation lifetime spectroscopy (PALS), a pulsed slowpositron beam system having both a high pulsing eAciency and a good time resolution is now under development. The time resolution, which is defined by the full width at half maximum (FWHM), and the pulsing eAciency of this system were achieved to be 0.54 ns and 50%, respectively. The lifetime of ortho-positronium (o-Ps) in low-density polyethylene (LDPE), obtained by PALS using our new pulsing system, agreed with that obtained by a conventional

Characterization of polymer sub-surface using slow positron beam

A new pulsed mono-energetic slow positron beam as well as the conventional positron annihilation lifetime spectroscopy (PALS) have been applied to study the sub-surface and the bulk of epoxy polymer. Significant changes of o-Ps parameters were found at a short distance from the surface. The lifetime of o-Ps was observed to decrease with increasing the positron implantation depth, while its intensity increased. The temperature effect on o-Ps parameters at sub-surface was also investigated. The glass transition temperature for the sub-surface was lower than that for the bulk. Furthermore, the thermal expansion coefficient of the sub-surface was found smaller than that of the bulk.

Positron-Annihilation Studies of Frank-Kasper-Type Quasicrystals of the Mg–Zn–Ga–Al and the Mg–Zn–Rare-Earth System

We have performed positron-annihilation experiments for a Frank-Kasper (F-K)-type icosahedral quasicrystal, 2/1 and 1/1 approximant crystals of Mg–Zn–Ga–Al, and the stable F-K-type icosahedral quasicrystals of Mg–Zn–RE (RE=rare-earth metal, Y, Ho and Gd) alloys. It is shown that a high density of vacant centers in triacontahedral clusters exists in both icosahedral and approximant phases of Mg–Zn–Ga–Al. Furthermore, in the present study, we report a very interesting finding that stable non-Al-based quasicrystals such as the icosahedral quasicrystal Mg–Zn–RE system also contain a high density of structural vacancies, similar to the stable Al-based quasicrystals. In particular, it is estimated experimentally that the structural vacancy densities for icosahedral quasicrystal Mg34Zn58Y8 and Mg40Zn52Ho8 are 3 ×1020 and 4 ×1020 cm-3, respectively. These results provide important information on the structural modeling of the stable icosahedral quasicrystal Mg–Zn–RE, which is a recent research topic of the icosahedral quasicrystals.

Development on a pulsed slow-positron beam: Moderator and bunching signal waveform

A pulsed slow-positron beam has been constructed for applications to polymer films. A new setup of the moderator system and a new design for the bunching signal waveform are introduced. The results show that such a kind of moderator appears to be good one, and that a new waveform for the bunching is very practical. The current pulsed slow-positron beam can be used to characterize polymer films or porous thin films.

Development of a pulsed slow-positron beam using time-varying pulsing-bias

Abstract In order to study the near-surface characterization of polymers using positron-annihilation lifetime spectroscopy (PALS), a pulsed slow-positron beam system is under development. Positrons emitted from 22 Na were injected periodically into a target by adjusting the time of flight of positrons between the moderator and the target using a time-varying electric field. By increasing the slope of the pulsing bias, the time resolution of this system, a full width at half maximum (FWHM) of 0.82 ns has been achieved. The lifetimes of ortho-positronium (o-Ps) in high-density polyethylene (HDPE) and polytetrafluoroethylene (PTFE) were obtained as 2.5 and 4.1 ns, respectively, by PALS using our new system. These values agree with those obtained by the conventional PALS.

Carbon-implanted ultra-high molecular-weight polyethylene studied by a pulsed slow-positron beam

The pulsed slow-positron beam (PSPB) technique was applied to study the structural changes in ultra-high molecular-weight polyethylene (UHMWPE) after carbon implantation, and the unimplanted material as well. A simple macroscopic model was introduced to explain the PSPB results. In the case of carbon-implanted UHMWPE, a three-layer model was used, and a very good agreement between the experimental and calculated results was demonstrated. A low-porosity layer, the region of which was in satisfactory agreement with the carbon ions stopping region, calculated by the TRIM code, was detected. An increase in the free volume and a decrease in the hole sizes were observed for a subsurface layer.

Application of a pulsed slow-positron beam to polymers

Pulsed slow positrons were produced using a time-varying moderator bias with an interval of 82 ns; 97% of the positrons were compressed within 2 ns width at the target position. Both the positron annihilation lifetime and Doppler broadening of the positron annihilation radiation (DBPR) of polytetrafluoroethylene (PTFE) were measured as a function of the incident energy of slow positrons. It was shown that the lifetime and intensity of the long-lived component of positron annihilation are independent of the positron incident energy above 1.2 keV. However, the width of the Doppler-broadened annihilation γ-ray increased in the energy region below 1.2 keV.